1. Field of the Invention
The present invention generally relates to a method for controlling air/fuel ratio of fuel supply system for an internal combustion engine.
2. Description of Background Information
In order to supply proper amount of fuel to an internal combustion engine, fuel supply systems are generally in use in which the fuel is supplied to the engine in accordance with a fuel supply control signal and uses a fuel supply device such as a fuel injector or injectors. The fuel supply control signal is calculated from a basic value of the fuel supply which is repeatedly derived using a basic engine parameter, such as a pressure in the intake passage of the engine, in synchronism with the engine rotation. To derive an actual fuel supply amount, an increment compensation or a decrement compensation is effected to the basic value in response to auxiliary engine parameters, such as an engine coolant temperature, or a parameter indicative of transitional changes of the engine. The fuel injector is actuated for each time duration corresponding to the thus derived actual fuel supply amount.
In the case of this type of fuel supply control system, if a three-way catalytic converter is provided in an exhaust system of the engine for the purification of the exhaust gas, the air-fuel ratio of the mixture to be supplied to the engine is to be controlled around a stoichiometric value (14.7:1, for instance) because the operation of the three-way catalytic converter is optimized when the air-fuel ratio of the mixture is controlled at the stoichiometric value.
For satisfying this requirement, an arrangement is generally utilized in which oxygen concentration in the exhaust gas is detected as one of the engine parameters by means of an oxygen concentration sensor (abbreviated as O.sub.2 sensor hereinafter) provided in the exhaust system. The basic value is corrected in accordance with an output signal of the O.sub.2 sensor so as to effect a feedback control operation through which the air-fuel ratio of the mixture supplied to the engine is controlled at the stoichiometric value.
However, the feedback control of the air-fuel ratio is not always effected. During a predetermined operating condition of the engine such as in a state where the engine coolant temperature is low, or a high load operating condition of the engine, the air-fuel ratio is enriched by an open loop control where the air-fuel ratio is determined without regard to the output signal of the O.sub.2 sensor.
In addition, the enrichment of the air-fuel ratio by increasing the fuel supply amount during the high load operation of the engine has an advantageous effect such that the engine is protected by the cooling effect of the enriched mixture during a high speed operation of the engine by which knocking and temperature rise of the cylinder wall are prevented. In order to sense a control range for the enrichment of the air-fuel ratio in terms of the engine rpm and the pressure within the intake pipe of the engine, a reference level is utilized in that a reference level of the vacuum is changed stepwisely with the variation of the engine speed. Specifically, the reference level of the pressure decreases as the engine speed increases.
On the other hand, a control method to be effected in a fuel supply system having this type of air-fuel ratio control operation is disclosed in Japanese Patent provisional publication No. 59-548. In this method, when the fuel supply amount exceeds a predetermined value, it is detected that the engine is operating under a high load condition and the open loop control is selected instead of the feedback control of the air-fuel ratio.
However, when the control range for increasing the fuel supply amount during the high load operating condition is detected in terms of the engine rotational speed and the pressure in the intake passage and the reference level of the pressure varies depending on the engine rotational speed as already described, there was a problem of the generation of an undesirable control range where necessary enrichment of the air-fuel ratio does not occur since the feedback control is selected because the derived fuel supply amount is smaller than the predetermined value. In such undesirable control range, the drivablity of the engine will be deteriorated.